The rock-paper-scissors lizard

Side-blotched lizards cycle through three different colour patterns and behaviours in an evolutionary game of rock-paper-scissors.

Side-blotched lizard
Judy Gallagher, CC BY 2.0, via Wikimedia Commons

The common side-blotched lizard is found across the western US and northern Mexico. If you went out and found a population of side-blotched lizards right now (go ahead, I’ll wait), you may find three different forms of lizard males: one group with orange-coloured throats, one with blue throats, and one with yellow throats.

One of those forms will outnumber the others by a significant margin; for the purposes of this example, let’s say there are many more orange-throated lizards. Clearly, evolutionary processes have favoured orange throats.

However! If you go back to that same population a few years later, orange- and blue-throated lizards will be outnumbered by male lizards with yellow throats. And then a few years after that, blue throats will be the new rage. Return again, and we’re back to the dominance of orange-throated lizards. What’s going on here?

In fact, it’s a game of rock-paper-scissors.

Orange-throated male side-blotched lizards are brimming with testosterone. They round up a bunch of female lizards and patrol a large personal territory. Yellow-throated lizards, in contrast, like to sneak into other lizards’ territories in order to mate with those females. Because orange-throated lizards have such a wide territory to patrol, it’s easy for yellow-throated lizards to sneak in and out – so the next generation will have many more yellow-throated lizards because of these illicit liaisons.

Blue-throated lizards also establish territories, but they are much smaller and typically only include a single female. All those sneaking yellow-throated lizards cannot get into the blue territories… so a yellow-dominant generation is often followed by one full of blue-throated lizards.

Then the orange-throated lizards re-enter the equation: because they’re large and aggressive they can bully out the blue-throated lizards. So a blue-dominant generation is followed by an orange-throated one.

To put it plainly: in this evolutionary dance, yellow beats orange, blue beats yellow, and orange beats blue. Rock, paper, scissors.

The technical term for all this, by the way, is frequency-dependent selection. The success of each form of the side-blotched lizard is tied to the quantity of each form in the population – as its frequency increases, another form is able to more effectively exploit its weaknesses, so its frequency decreases.

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